In holographic data storage, digital data is stored by recording the interference pattern produced by the superposition of two coherent laser beams. Where one beam, the so-called “object beam”, is modulated by a spatial light modulator (SLM) and carries the information/data to be recorded, the second beam serves as a reference beam. The interference pattern leads to a modification of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam, the so called reconstructed object beam that is detected by a detector array.
One advantage of holographic data storage is an increased data capacity per volume and a higher data transfer rate. Instead of storing single bits, as it is known from conventional media such as CD or DVD, data is stored as data pages. Typically, a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve an increased data rate in addition to the increased data storage density.
Conventional optical storage media only use a single or a few 2-dimensional layers for data storage. Contrary, in holographic data storage media the volume of the medium is used for storing data instead of a layer. Further, the volume of the material used for data storage is not completely exhausted when data is written to it. The advantage of holographic data storage is the possibility to store multiple data in the same volume. This is achieved e.g. by changing the angle between the object beam and the reference beam, called angle multiplexing. A further option, called shift multiplexing, applies a slight shift between subsequent holograms, which do overlap each other for the most part. Shift multiplexing is performed e.g. by movement of the storage material.
However, the reconstructed object beam suffers under interference effects with stray light inside the optical setup of the apparatus for holographic data stage. The result of such interference effects is a disturbed reconstructed object beam that in turn leads to data readout errors.